1. Field of the Invention
The present invention relates to a zoom lens, more specifically, to the zoom lens utilizing a plastic aspheric lens.
2. Description of the Prior Art
In recent years, a market for a digital camera is expanding greatly, and the user""s demand for the digital camera have been ranged in scope accordingly. That is, needless to say that not only a high image quality and a miniaturization are demanded, but also the demand for a weight saving of a zoom lens becomes greater in recent years.
There are considered various types of zoom lenses for the digital camera. As a type suited for widening an angle of view and for the miniaturization, there is a zoom lens of type which has a first group of lenses having a negative focal length, a second group of lenses having a positive focal length and a third group of lenses having a positive focal length which are arranged subsequently from a side of an object side to a side of an image, and has an aperture stop disposed at the object side of the second group of lenses that moves integrally with the second group of lenses, and in a change of magnification from a short focal end to a long focal end, the second group of lenses monotonously moves from the image side to the object side and the first group of lenses moves so as to correct a displacement of a position of image plane in accordance with the change of magnification.
For example, in Japanese Patent Laid Open No. H10-039214, there is disclosed a small-size zoom lens which a position of an exit pupil can be separated from an image plane sufficiently, has the wide angle of view and is blight, has a good performance, and yet, capable of carrying out a large change of magnification ratio. According to the small-sized zoom lens disclosed in Japanese Patent Laid Open No. H10-039214, it is constituted by arranging a first group of lenses to a third group of lenses subsequently from the object side to the image side, and the first group of lenses has a negative refractive power, the second group has a positive refractive power and the third group of lenses has the positive refractive power. It also has an aperture stop at the object side of the second group of lenses that moves integrally with the second group of lenses when carrying out a zooming, and the third group of lenses is a fixed group relating to the zooming. At the time of carrying out the zooming from a wide angle end to a telephoto end, the first group of lenses is moved to the image side on an optical axis firstly and then a direction of its movement is reversed to the object side on the way so that the first group of lenses is moved to the image side in such a manner as to draw a convex-like arc shape to correct a displacement in an focusing position, and the second group of lenses is moved monotonously to the object side on the optical axis to carry out the change of magnification.
Also, in Japanese Patent Laid Open 2000-267009, a small sized, high image quality and inexpensive zoom lens which arranges a plastic lens for a zoom lens which has a negative-positive two-element zoom lens effectively and which is particularly suited for a digital still camera, is disclosed. According to the zoom lens disclosed in Japanese Patent Laid Open 2000-267009, at least one of lenses constitutes each group of lenses in a zoom lens having negative-positive-positive three-element lens seen from the object side is the plastic lens, and it also has a structure which a following conditional formula is satisfied: xe2x88x920.8 less than Cpxc3x97(Nxe2x80x2xe2x88x92N)/xcfx86W less than 0.8xe2x88x920.45 less than M3/M2 less than 0.90 (where xcfx86T/xcfx86W greater than 1.6) where Cp is a curvature of the plastic lens, xcfx86W is a power of entire system at the wide angle end, N and Nxe2x80x2 are the object side of an aspheric surface and an refraction index of d-line of medium at the image side, M2 and M3 show amounts of movements of the second group of lenses and the third group of lenses respectively (using the wide angle end as the base and setting the object side as xe2x88x92), and xcfx86T is the power of entire system at the telephoto end.
The above mentioned Japanese Patent Laid Open No. H10-039214 was filed at the earliest time as the above zoom lens of such type, and a basic structure of the above zoom lens of such type is all disclosed. However, since all the lenses are made from glass, it cannot be said that the sufficient weight saving is achieved.
Also, Japanese Patent Laid Open 2000-267009 provides the zoom lens utilizing a plastic aspheric lens. However, the second group of lenses having a function of the change of magnification and an imaging function which are most influential on an aberration correction function is a triplets type in the embodiment described in Japanese Patent Laid Open 2000-267009, therefore there is a problem that it has no sufficient aberration correction function.
The present invention has been made in view of the aforementioned problems, and therefore it is an object of the present invention to provide a zoom lens which is small sized, light weighted and has a wide angle of view by using a plastic aspheric lens while having high performance equivalent to or more than conventional zoom lenses.
To accomplish the aforementioned object, a zoom lens according to a first aspect of the present invention comprises a first group of lenses having a negative refractive power, a second group of lenses having a positive refractive power and a third group of lenses having the positive refractive power, and an aperture stop provided at an object side of the second group of lenses. In addition, a first positive lens, a negative lens, a second positive lens and a third positive lens are subsequently arranged from the object side to the image side in the second group of lenses. Furthermore, when a zooming from a short focal end to a long focal end is carried out, the second group of lenses is monotonously moved from an image side to the object side, and the first group of lenses is moved so as to correct a displacement of a position of image plane in accordance with the zooming, and at least the third positive lens positioned nearest to the image side is a plastic aspheric lens.
Also, according to a second aspect of the present invention, the negative lens is formed by a negative lens in a meniscus shape which faces its convex surface toward the object side. In addition, the second positive lens is formed by a positive lens in a meniscus shape which faces its convex surface toward the object side.
In the zoom lens of the present invention, the first group of lenses to the third group of lenses are arranged subsequently from the object side to the image side. The first group of lenses has the xe2x80x9cnegative refractive powerxe2x80x9d, the second group of lenses has the xe2x80x9cpositive refractive powerxe2x80x9d and the third group of lenses has the xe2x80x9cpositive refractive power.xe2x80x9d The aperture stop provided at the object side of the second group of lenses moves integrally with the second group of lenses when carrying out the zooming. In addition, when carrying out the zooming from a wide angle end to a telephoto end, the first group of lenses is moved to the image side on an optical axis firstly and then a direction of its movement is reversed to the object side on the way so that the first group of lenses is xe2x80x9cmoved to the image side in such a manner as to draw a convex-like arc shapexe2x80x9d to correct a displacement in an focusing position, and the second group of lenses is moved monotonously to the object side on the optical axis, to carry out the change of magnification. Also, since the aperture stop is moved integrally with the second group of lenses when carrying out the zooming, the movement of the second group of lenses is not hindered by the aperture stop. The most characteristic of the present invention is that at least the third positive lens positioned nearest to the image side is the plastic aspheric lens.
According to this invention, because the plastic aspheric lens is used for the second group of lenses with a conventional structure of the zoom lens, the small sized camera (portable information terminal apparatus) which is lighter weighted and more convenient to use than the conventional ones and capable of widening the angle of view can be realized while having the high performance and in small size equivalent to or more than the conventional ones.
Also, according to a third aspect of the present invention, a following conditional formula is satisfied:
0.08 less than f2/f2p less than 0.8
if a focal length of the second group of lenses is f2, and the focal length of the third positive lens is f2p.
By providing the second group of lenses to have two pieces of the positive lenses to the image side, power can be weakened per one piece in the zoom lens of the present invention. Generally, although the plastic lens is known to have a tendency that the position of image plane or the like is easily be displaced by an environmental variation, the plastic lens in the present invention becomes difficult to be influenced by the environmental variation by introducing such lens having relatively weak power, as a result, the widening of angle of view in photographing angle of view and the weight saving of a lens system can be accomplished together. Here, a focal length of the plastic aspheric lens is desired to satisfy the conditional formula described in the present invention.
According to this invention, since the focal length of the plastic aspheric lens is designed to satisfy the aforementioned conditional formula, the zoom lens which has the high performance and difficult to be influenced by the environmental variation even more can be provided.
Also, according to the forth aspect of the present invention, at least the negative lens of the second group of lenses and the second positive lens are jointed.
In order to reduce performance degradation by an error in assembling the lens in the zoom lens of the present invention, it is preferred to join the negative lens of the second group of lenses with the second positive lens in the image side thereof.
According to this structure, since the negative lens of the second group of lenses and the second positive lens in the image side thereof are jointed, the zoom lens having simpler structure with less performance degradation can be provided.
Also, according to a fifth aspect of the present invention, a following conditional formula is satisfied:
0.8 less than Rc/Ymax less than 1.2
if a radius of curvature in a jointed surface of the second group of lenses is Rc, and a maximum image height is Ymax.
Although the aberration in directions mutually counteracting at a surface of the image side of the negative lens and at a surface of the object side of the second positive lens is largely generated and therefore degradation of image performance by a relative de-centering of the two lenses is large, such degradation of image performance can be avoided by joining the negative lens with the second positive lens. In order to attain the zoom lens having the high performance even more when joining the negative lens with the second positive lens, it is desired to satisfy the conditional formula in the present invention.
According to this invention, since the design is made to satisfy the conditional formula, the zoom lens having the high performance and simple structure can be provided.
Also, according to a sixth aspect of the present invention, the aperture stop provided at the object side of the second group of lenses moves integrally with the second group of lenses, and at least a surface of the second group of lenses that is nearest to the object side is an aspheric surface.
To provide the zoom lens of the present invention to be further simplified and in high performance even more, it is desired to provide the aperture stop which moves integrally with the second group of lenses at the object side of the second group of lenses and set at least the object side surface in the lens at nearest to the object side of the second group of lenses to be aspheric surface.
According to this invention, because the aperture stop which moves integrally with the second group of lenses is provided and at least the object side surface in the lens at the nearest to the object side of the second group of lenses is set to be aspheric surface, the zoom lens which an aspheric aberration is less can be provided, as a result, the camera (portable information terminal apparatus) having even higher image quality can be realized.
Also, according to a seventh aspect of the present invention, at least one piece of plastic aspheric lens, whose both surfaces thereof are aspheric surfaces, is provided.
Although more aspheric surfaces may be provided in order to improve the aberration correction function even more, if a lens which has the aspheric surfaces on both sides thereof are introduced by using a high degree of freedom in processing the plastic lens, great effect in terms of efficiency can be attained.
According to this invention, because at least one piece of the plastic aspheric lens is provided, the zoom lens which is superior in the aberration correction function can be provided.
Also, according to an eighth aspect of the present invention, the first group of lenses is provided with a plastic aspheric lens, and the third group of lenses is provided with a plastic aspheric lens.
A large number of plastic lenses may be used in order to pursue more weight saving of the zoom lens in the present invention, but there is no so much choice for the plastic which can be used as the lens compared with an optical glass in terms of a refraction index and dispersion. Accordingly, the aberration correction function deteriorates if the excessive plastic lenses are used in the same group of lenses rather than raising the aberration correction function. To take a balance between the weight saving and the aberration correction function efficiently while avoiding the deterioration in the aberration correction function, it is preferred to adopt a method in which the plastic aspheric lenses distribute to each of the group of lenses.
More specifically, in one of the zoom lens in the present invention, the third positive lens which the second group of lenses provides is the plastic aspheric lens, the first group of lenses provides at least one plastic aspheric lens, and the third group of lenses provides at least one plastic aspheric lens.
According to this invention, since the plastic aspheric lenses are distributed to each of the group of lenses, even lighter zoom lens can be provided.
According to a ninth aspect of the present invention, the zoom lens described in the above first aspect is provided as a photographing optical system.
According to this invention, since the zoom lens which can be miniaturized even more and has the high performance equivalent to or more than the conventional ones is used for a photographing optical system, the camera which is small in size and has the high image quality can be provided, as a result, the user can photograph the high quality image by using the camera which is superior in portability.
Also, according to a tenth aspect of the present invention, the zoom lens described in the first aspect is provided as a photographing optical system of a camera function section.
According to this invention, because the zoom lens which can be miniaturized even more and has the high performance equivalent to or more than the conventional ones is used for a photographing optical system of the camera function section, the portable information terminal apparatus which is small in size and has the high image quality can be provided, as a result, the user can photograph the high quality image by using the portable information terminal apparatus which is superior in portability, and can transmit the photographed image to outside.